An urban utility experiences approximately 1,500 failures on its network feeders each year. Each feeder outage duration is directly proportional to the risk of customer interruption and the stress experienced by other feeders and transformers in the network. The defective component must remain out of service during repair and/or replacement. This means that the whole feeder remains out of service or a live end cap must be installed to separate the main feeder from the spur containing the defect. If a live end cap is installed, the feeder must be de-energized a second time to reconnect the required spur. This second outage is usually scheduled as soon as possible to restore the system to normal full capability. However, the perceived risk of scheduling the entire feeder out of service to pick-up a small spur is very large, especially during the summer or other high load periods.
Encapsulated switch assemblies with sub-atmospheric or vacuum type circuit interrupters for electric power circuits and systems are well known in the art, such as is shown in U.S. Pat. Nos. 4,568,804; 3,955,167; 3,471,669; 3,812,314; and 2,870,298. In some prior art switch assemblies and circuit breakers, a pair of coacting contacts, one fixed and the other movable, are provided for controlling and interrupting current flow. The contacts are provided in a controlled atmosphere contact assembly which may include a relatively fragile glass or ceramic housing, commonly referred to as a “bottle” for housing the contacts. A metal bellows may be provided on one end of the bottle, and the movable contact is linked to the inside of the bellows. An operating rod attached to the outside of the bellows can be moved so as to move the movable contact inside the bottle. The interior of the bottle is maintained under a controlled atmosphere, such as air or another gas under a low subatmospheric pressure, to protect the contacts from damage caused by arcing when the contacts are opened and closed. The glass or ceramic wall of the bottle provides a permeation-resistant enclosure which maintains the controlled atmosphere for the life of the device.
More recently, elastomer-insulated switch housings using a controlled atmosphere contact assembly have been introduced for underground power distribution systems and other, similar applications. Switches for use in such applications must meet several demanding requirements. Those parts of the switch assembly connected to line voltage during use, including the contact assembly and operating rod, must be encased in a solid insulating housing having dielectric strength sufficient to withstand the maximum voltage which may be imposed on the system, which may be tens of thousands of volts for a distribution-level system. For safety, the insulating housing should be covered with a conductive layer that can be grounded. The switch should be operable from outside of the dielectric housing, without opening the housing and should be capable of withstanding many years of exposure to temperature extremes, water and environmental contaminants.
Elastomers such as EPDM (ethylene propylene diene monomer) combine high dielectric strength with excellent resistance to the effects of ozone and corona discharge. These elastomers can also provide good physical properties such as abrasion resistance, and can be molded at reasonable cost. Additionally, these elastomers can be compounded with conductive additives and molded to provide an electrically conductive grounding layer integral with the dielectric housing. For these and other reasons, elastomers molded and vulcanized under heat and pressure, such as EPDM, have been almost universally adopted as materials of construction for the housings used in many underground electrical distribution systems.
An important feature in such switch assemblies and circuit breakers is the ability to visually determine the switched condition of the contacts. This is obviously important for safety reasons in that power must be disconnected before accessing or repairing a switch branch. U.S. Pat. No. 4,568,804 discloses a high voltage vacuum type circuit interrupter having a one-piece ceramic insulating housing connected to a two-part metallic base. The base encloses a solenoid operated toggle mechanism that controls and operates movement of a switch contact to open and close the switch. The base further includes a sight glass or lens secured to the bottom of the base, through which a switch position indicator is visually discernible.
One drawback with the circuit interrupter disclosed in the '804 patent is its size and complexity in manufacture. Another drawback relates to the fact that the position indicator is located at the toggle mechanism away from the switch contacts. In other words, while the position indicator of the '804 patent may show the condition of the toggle mechanism, there is no provision for visually confirming whether the switch contacts are indeed in contact or separated.
As mentioned above, another concern with such switch assemblies is flashover or arcing of the electric current between switch contacts. Aside from safety concerns, such arcing causes damage to the contacts and the surrounding housing. While efforts to reduce arcing by enclosing the contacts in an evacuated chamber or by insulating the contacts with an arc quenching gas or oil have proven somewhat successful, arcing still occasionally occurs in the field. Additionally, vacuum chambers typically require a housing made from ceramic. Air insulation chambers are generally very large. Chambers filled with SF6 arc quenching gas must be hermetically sealed and maintained to ensure no leakage and insulating oils have been found to fail catastrophically resulting in injury to people and damage to equipment.
Yet another problem with high current switches described above is related to electromagnetic fields which generate undesirable bending forces. In particular, the feeder contact is arranged generally at a 90° angle to the switches current carrying contact pin. These electromagnetic forces are produced on the current carrying members causing a cantilever bending movement at the connection interface.
Accordingly, it is desirable to provide a simply constructed, electrically insulated, switch assembly having direct visible verification of open or closed contacts. It is further desirable to provide such a switch assembly that minimizes the possibility of arcing between electrical contacts and provides good electrical continuity through the switch assembly.